This is a major release of dex and represents a complete rewrite of the project. The new version has significantly simplified dex's deployment and management story while rethinking and slimming down the implementation.
A full writeup of changes can be found here: https://github.com/coreos/dex/blob/master/Documentation/v2.md
Though v1.0.0 was never tagged, this branch is such a significant departure from previous development that it seems appropriate to call it a v2.